This invention relates to the measurement of physical quantities by means of the backscattering of light. It specifically relates to an apparatus for time-resolved optical backscatter measurement (OTDR=Optical Time Domain Reflectometry) at optical waveguides, in particular for space-resolved temperature measurement, comprising a light source, which emits light of a specified wavelength, an optical waveguide, first means for coupling the light from the light source into the optical waveguide, second means for coupling the backscattered light, which includes components originating from a Rayleigh scattering and a Raman scattering, out of the optical waveguide, and third means for measuring the backscattered Raman component coupled out of the optical waveguide.